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Chap 15 Application Layer

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  1. Chap 15 Application Layer Andres, Wen-Yuan Liao Department of Computer Science and Engineering De Lin Institute of Technology andres@dlit.edu.tw http://www.cse.dlit.edu.tw/~andres

  2. Overview • Client-Server • Redirectors • Domain Name System • E-mail/Telnet/FTP/HTTP

  3. Basics of the Application Layer • Application processes • Direct network applications • Indirect network support • Making and breaking a connection

  4. Application processes • Supports the communicating component of an application • The OSI layer closest to the end system • Does not provide services to any other OSI layer

  5. Application processes • Direct interface • Browser, e-mail, FTP, Telnet • Indirect interface • Word processors, spreadsheets, presentation managers with a network redirector

  6. Direct network applications • Client-server applications (most) • FTP, web browsers, and e-mail • The client side • On the local computer • The requestor of the services • The server side • On a remote computer • Provides services

  7. WWW • Browsers (NN and IE) are probably the most commonly used network applications • An easy way to understand a Web browser is to compare it to a television remote control

  8. Indirect network support • If a client wants to save a file from a word processor to a network server, the redirector enables the word processing application to become a network client

  9. Redirector • A protocol that works with computer operating systems and network clients instead of specific application programs

  10. Redirectors • Apple File Protocol • NetBIOS Extended User Interface (NetBEUI) • Novell IPX/SPX protocols • Network File System (NFS) of the TCP/IP protocol suite

  11. Redirector • Allows a network administrator to assign remote resources to logical names on the local client (F:, G:, … )

  12. Advantages • The applications on the client never have to recognize the network • Expand the capabilities of non-network software

  13. Making/breaking a connection • Web • The connection was maintained just long enough to download the current Web page • Printer • The connection was maintained just long enough to send the document to the print server

  14. Domain Name System • Problems with using IP addresses • The domain name server

  15. Problems with using IP addresses • A domain is a group of computers that are associated by their geographical location or their business type • A domain name is a string of characters and/or numbers • More than 200 top-level domains

  16. Generic names • .edu - educational sites • .com - commercial sites • .gov - government sites • .org - non-profit sites • .net - network service

  17. The domain name server • Domain name -> IP address • The DNS system is set up in a hierarchy that creates different levels of DNS servers

  18. Network Applications • Internet applications • E-mail message • DNS function

  19. Internet applications • The WWW uses the HTTP protocol • Remote access programs : Telnet protocol • E-mail programs : POP3 application layer protocol for electronic mail • File utility programs : FTP protocol

  20. E-mail message • The first is to send the e-mail to the user’s post office • The second is to deliver the e-mail from that post office to the user’s e-mail client

  21. Application Layer Examples • Telnet • File transfer protocol • Hypertext transfer protocol

  22. Telnet • All processing and storage take place on the remote computer • Works mainly at the top three layers of the OSI model • Application, presentation , session layers

  23. FTP • Be designed to download files (e.g. receive from the Internet) or upload files (e.g. send to the Internet)

  24. Hypertext transfer protocol • The fastest growing and most used part of the Internet • A Web browser is a client-server application • Requires both a client and a server component in order to function

  25. Hyperlink • A hyperlink is an object (word, phrase, or picture) on a Web page that, when clicked, transfers you to a new Web page

  26. Summary • Direct and indirect network applications • The domain name system • Telnet, FTP and HTTP